280 research outputs found
Bias and temperature dependence of the noise in a single electron transistor
A single electron transistor based on Al-AlO_x-Nb tunnel junctions was
fabricated by shadow evaporation and in situ barrier formation. Its output
current noise was measured, using a transimpedance amplifier setup, as a
function of bias voltage, gain, and temperature, in the frequency range 1...300
Hz. The spot noise at 10 Hz is dominated by a gain dependent component,
indicating that the main noise contribution comes from fluctuations at the
input of the transistor. Deviations from ideal input charge noise behaviour are
found in the form of a bias dependence of the differential charge equivalent
noise, i. e. the derivative of current noise with respect to gain. The
temperature dependence of this effect could indicate that heating is activating
the noise sources, and that they are located inside or in the near vicinity of
the junctions.Comment: 16 pages, 9 figures (EPS
Two-dimensional arrays of low capacitance tunnel junctions: general properties, phase transitions and Hall effect
We describe transport properties of two-dimensional arrays of low capacitance
tunnel junctions, such as the current voltage characteristic and its dependence
on external magnetic field and temperature. We discuss several experiments in
which the small capacitance of the junctions plays an important role. In arrays
where the junctions have a relatively large charging energy, (i.e. when they
have a low capacitance) and a high normal state resistance, the low bias
resistance increases with decreasing temperature and eventually at very low
temperature the array becomes insulating even though the electrodes in the
array are superconducting. This transition to the insulating state can be
described by thermal activation. In an intermediate region where the junction
resistance is of the order of the quantum resistance and the charging energy is
of the order of the Josephson coupling energy, the arrays can be tuned between
a superconducting and an insulating state with a magnetic field. We describe
measurements of this magnetic-field-tuned superconductor insulator transition,
and we show that the resistance data can be scaled over several orders of
magnitude. Four arrays follow the same universal function. At the transition
the transverse (Hall) resistance is found to be very small in comparison with
the longitudinal resistance. However, for magnetic field values larger than the
critical value.we observe a substantial Hall resistance. The Hall resistance of
these arrays oscillates with the applied magnetic field. features in the
magnetic field dependence of the Hall resistance can qualitatively be
correlated to features in the derivative of the longitudinal resistance,
similar to what is found in the quantum Hall effect.Comment: 29 pages, 16 eps figures, uses aipproc.sty and epsfig.sty,
contribution to Euroschool on "Superconductivity in Networks and Mesoscopic
Systems", held in Siena, Italy (8-20 september 1997
Gain Dependence of the Noise in the Single Electron Transistor
An extensive investigation of low frequency noise in single electron
transistors as a function of gain is presented. Comparing the output noise with
gain for a large number of bias points, it is found that the noise is dominated
by external charge noise. For low gains we find an additional noise
contribution which is compared to a model including resistance fluctuations. We
conclude that this excess noise is not only due to resistance fluctuations. For
one sample, we find a record low minimum charge noise of qn = 9*10^-6
e/sqrt(Hz) in the superconducting state and qn = 9*10^-6 e/sqrt(Hz) in the
normal state at a frequency of 4.4 kHz.Comment: 10 pages, LaTex 2.09, 4 figures (epsfig
Predicting project performance using pre-construction performance indicators - A case study evaluation
Predicting the outcome of a construction project largely relies on estimated targets of time and cost. Still, hitting the targets does not mean that the project is a success on all performance levels. Here, a retrospective case study was undertaken on a construction project identified as a successful project by the partners involved. The purpose of the study was to validate conceptual design indicators of a high-performance construction project as reported in the literature, by answering the following research questions: "What characterizes the dialogue between the different disciplines; (2) What is the dialogue about; and (3) When in the process do questions arise?" Findings indicate that the interprofessional dialogue within the project team was well established. The paper introduces a discussion that the dialogue benefited from the collaborative project environment as well as the early design intent. Further, the study also suggests that the interprofessional dialogue supported a sound project team development
Revision of the Eocene \u27Platyrhina\u27 species from the Bolca Lagerstätte (Italy) reveals the first panray (Batomorphii: Zanobatidae) in the fossil record
The fossil-Lagerstätte of Bolca (Italy) is well known for the diversity and exquisite preservation of its bony and cartilaginous fishes documenting tropical shallow-water marine environments associated with coral reefs in the western Tethys during the early Eocene. In this study, the taxonomic, systematic and phylogenetic position of two batoid species traditionally assigned to the living thornback ray genus Platyrhina is re-evaluated. †Platyrhina bolcensis Heckel, 1851 is recognized as a separate species of the Platyrhinidae because of its plate-like antorbital cartilage with an irregular outline and a small horn on the nasal capsules. Also, the rostral cartilage does not reach the anterior border of the disc. Support for the placement of this species within the new genus †Eoplatyrhina gen. nov. is based on a combination of morphological and meristic features (e.g. nasal capsules at right angles to the rostrum; large space between the hyomandibulae and mandibular arch; approximately 132 vertebral centra; 15–16 rib pairs; 81–87 pectoral radials; 18–21 pelvic radials; short, straight and stout claspers; 40–50 caudal-fin radials; thorns absent). A second species, †Platyrhina egertoni (De Zigno, 1876), is more closely related to the living panray Zanobatus than Platyrhina and is assigned here to †Plesiozanobatus gen. nov. because of a combination of characters that support its placement within the family Zanobatidae (tail stout and short, distinctly demarcated from disc; two dorsal fins and complete caudal fin; small dermal denticles and scattered thorns covering disc and tail; rostral cartilage absent; nasal capsules without horn-like processes; mesopterygium absent). The systematic position of a third taxon, †Platyrhina gigantea (Blainville, 1818), is currently impossible to establish due to the poor preservation of the only known specimen, and therefore we propose to consider it a nomen dubium. Palaeoecological and biogeographic features of the Eocene platyrhinids and zanobatids from Bolca are also discussed
Giant lasing effect in magnetic nanoconductors
We propose a new principle for a compact solid-state laser in the 1-100 THz
regime. This is a frequency range where attempts to fabricate small size lasers
up till now have met severe technical problems. The proposed laser is based on
a new mechanism for creating spin-flip processes in ferromagnetic conductors.
The mechanism is due to the interaction of light with conduction electrons; the
interaction strength, being proportional to the large exchange energy, exceeds
the Zeeman interaction by orders of magnitude. On the basis of this
interaction, a giant lasing effect is predicted in a system where a population
inversion has been created by tunneling injection of spin-polarized electrons
from one ferromagnetic conductor to another -- the magnetization of the two
ferromagnets having different orientations. Using experimental data for
ferromagnetic manganese perovskites with nearly 100% spin polarization we show
the laser frequency to be in the range 1-100 THz. The optical gain is estimated
to be of order 10^7 cm^{-1}, which exceeds the gain of conventional
semiconductor lasers by 3 or 4 orders of magnitude. A relevant experimental
study is proposed and discussed.Comment: 4 pages, 3 figure
Superconductivity in an Einstein Solid AxV2Al20 (A = Al and Ga)
A cage compound AxV2Al20 (Al10V), that was called an Einstein solid by Caplin
and coworkers 40 years ago, is revisited to investigate the low-energy, local
vibrations of the A atoms and their influence on the electronic and
superconducting properties of the compound. Polycrystalline samples with A =
Al, Ga, Y, and La are studied through resistivity and heat capacity
measurements. Weak-coupling BCS superconductivity is observed below Tc = 1.49,
1.66, and 0.69 K for Ax = Al0.3, Ga0.2, and Y, respectively, but not above 0.4
K for Ax = La. Low-energy modes are detected only for A = Al and Ga, which are
approximately described by the Einstein model with Einstein temperatures of 24
and 8 K, respectively. A weak but significant coupling between the low-energy
modes, which are almost identical to those called rattling in recent study, and
conduction electrons manifests itself as anomalous enhancement in resistivity
at around low temperatures corresponding to the Einstein temperatures.Comment: 12 pages, 5 figures, to be published in J. Phys. Soc. Jp
The Phylogeny of Rays and Skates (Chondrichthyes: Elasmobranchii) Based on Morphological Characters Revisited.
Elasmobranchii are relatively well-studied. However, numerous phylogenetic uncertainties about their relationships remain. Here, we revisit the phylogenetic evidence based on a detailed morphological re-evaluation of all the major extant batomorph clades (skates and rays), including several holomorphic fossil taxa from the Palaeozoic, Mesozoic and Cenozoic, and an extensive outgroup sampling, which includes sharks, chimaeras and several other fossil chondrichthyans. The parsimony and maximum-likelihood analyses found more resolved but contrasting topologies, with the Bayesian inference tree neither supporting nor disfavouring any of them. Overall, the analyses result in similar clade compositions and topologies, with the Jurassic batomorphs forming the sister clade to all the other batomorphs, whilst all the Cretaceous batomorphs are nested within the remaining main clades. The disparate arrangements recovered under the different criteria suggest that a detailed study of Jurassic taxa is of utmost importance to present a more consistent topology in the deeper nodes, as issues continue to be present when analysing those clades previously recognized only by molecular analyses (e.g., Rhinopristiformes and Torpediniformes). The consistent placement of fossil taxa within specific groups by the different phylogenetic criteria is promising and indicates that the inclusion of more fossil taxa in the present matrix will likely not cause loss of resolution, therefore suggesting that a strong phylogenetic signal can be recovered from fossil taxa
Observation of anisotropic effect of antiferromagnetic ordering on the superconducting gap in ErNi2B2C
The point-contact (PC) spectra of the Andreev reflection dV/dI curves of the
superconducting rare-earth nickel borocarbide ErNi2B2C (Tc=11 K) have been
analyzed in the "one-gap" and "two-gap" approximations using the generalized
Blonder-Tinkham-Klapwijk (GBTK) model and the Beloborod'ko (BB) model allowing
for the pair-breaking effect of magnetic impurities. Experimental and
calculated curves have been compared not only in shape, but in magnitude as
well, which provide more reliable data for determining the temperature
dependence of the energy gap (or superconducting order parameter) \Delta(T).
The anisotropic effect of antiferromagnetic ordering at T_N =6 K on the
superconducting gap/order parameter has been determined: as the temperature is
lowered, \Delta(T) decreases by 25% in the c-direction and only by 4% in the
ab-plane. It is found that the pair-breaking parameter increases in the
vicinity of the magnetic transitions, the increase being more pronounced in the
c-direction. The efficiency of the models was tested for providing \Delta(T)
data for ErNi2B2C from Andreev reflection spectra.Comment: 16 two column pages, 20 figs., will be published in Fiz. Nizk. Temp.
N10, 2010; V2: added - "Acknowledgement" & "Note added in proof
- …